It has been suggested to employ active adsorbent materials, such as activated carbon and aluminum oxide, to adsorb impurities in the filtration of water. One such system is disclosed in U.S. Pat. No. 4,208,281, issued to Haberer et al, and herein incorporated by reference.
In the Haberer et al system buoyant plastic particles, constituting the media of the filter, are coated with the active powder material prior to directing the influent to be treated upwardly through the system. This type of filter, wherein the influent is directed in an upward direction through the media, is referred to as an upflow filter. In order to maintain the buoyant media in proper position within the filter a fine mesh screen, having openings significantly smaller than the nominal size of the buoyant media, has been fixed above the media layer. Although this arrangement will successfully retain the buoyant media in the filter, the openings in the screen, due to their required small size for retaining the media, tend to become clogged very fast, both by buoyant media particles pressing against the surface of the screen and by contaminants (e.g. coagulated solids) in the influent. When the screen does become excessively clogged, it is necessary to physically detach it from the filter for either cleaning or replacement. Moreover, due to the smallness of the openings through the screen, such excessive clogging of the screen will tend to occur more frequently than is desirable for efficient filter operation.
Neptune Microfloc also employs buoyant particles (specific gravity&lt;1) in the adsorption media of an upflow clarifier forming part of a combination adsorption-gravity filter system presently being advertised under the trademark Trident. The clarifier is operated in an upflow mode, with the influent being directed upwardly through the buoyant media. Like the Haberer et al system, the Trident system requires the use of a retainer above the buoyant media. If a fine mesh screen is employed for this purpose, the same problems described above in connection with the Haberer et al system also will be encountered in the Trident system.
It also should be noted that, in the Haberer et al system, the flushing, or backwashing operation takes place by directing liquid wash water downwardly through the buoyant media, whereas in the Trident system the flushing or backwash cycle takes place by directing air and water upwardly through the buoyant media. If the backwashing water is dirty or impure (i.e. containing suspended solids in it), the backwashing operation in both systems can further clog the buoyant media-retaining screen. Moreover, due to the upward direction of the backwashing cycle in the Trident system, solids removed from the buoyant media may likewise be directed into the screen to clog it.